1. Field of the Invention
The present invention relates generally to tunable T-junctions and more particularly to tunable microstrips having multiple high frequency contactless tuning stubs and the use of such tunable microstrips to fabricate tunable T-junctions.
2. Description of the Prior Art
A microstrip is a type of electrical transmission line which can be fabricated using printed circuit board technology and is used to convey microwave-frequency signals. It consists of a conducting strip separated from a ground plane by a dielectric layer known as the substrate. For many applications, microstrips can be formed on pc boards from the metallization layers and the insulating body or substrate. Generally pc boards are made from copper and fiberglass, although for high frequency applications other materials with higher conductivity and higher dielectric values may be used. Microwave components such as antennas, couplers, filters, splitters and combiners can be formed from microstrips, the entire device existing as the pattern of metallization on the substrate. Microstrips are thus much less expensive than traditional waveguide technology, as well as being far lighter and more compact.
Tuning stubs and T-junctions are well known in the microwave industry. The first tuning stubs and T-junctions were mostly used in tuning devices for load pull measurements, in microwave antennas as a mechanism to control the resonant frequency, in matching networks to adjust the matching impedance, in filters as a means to adjust the center frequency, and in phase shifters to provide some degree of phase trimming. On the other hand, T-junctions alone have been widely used as either low loss splitters or combiners, but have seldom been tuned for any specific parameter.
T-junction power dividers/combiners are three port lossless devices and as such do not present isolation between the two output ports and only the sum port is matched. The lossless characteristic of T-junctions makes them suitable when there is a need for efficient power combining between two highly coherent power amplifiers. Efficiently performing such power combining relies on the loss properties of the dielectric material, the conductive trace's metallization technology as well as keeping as low as possible the amplitude and phase unbalances induced by the junction. Additionally, the sources to be combined must be as coherent as possible. T-junction devices have been implemented for decades in waveguide, stripline, coplanar and microstrip technologies among others and all of them follow the same principle of operation.
While microstrips and microstrip based T-junctions are easily manufactured on pc boards, achieving highly accurate parameter matching is not simple. The etching of microstrip circuit lines during manufacturing is frequently uneven due to manufacturing process variables and tolerances. The uneven etching leads to the circuit lines having different line widths. The uneven line widths cause impedance differences in the circuit lines and thus cause the insertion loss between the input port and output port to be different. The uneven line widths also cause amplitude unbalance between the input port and output port. Amplitude unbalance degrades the electrical performance of the combiner. The amount of amplitude unbalance is an important parameter in defining the performance of the combiner. Various tuning strategies have been used in high frequency stripline devices.
U.S. Pat. No. 7,015,772 entitled TUNABLE AMPLITUDE UNBALANCE STRIPLINE COMBINER teaches a tuned T-junction using windows cut into the ground plane of a stripline circuit to tune the signal amplitudes and thus minimize the amplitude unbalance. In this circuit tuning stubs were not used. There is no optimization for voltage breakdown or for the dynamic range of tuning. The circuit area required to realize this tuning method is further complicated by the need to remove significant amounts of material, thus increasing time and effort to tune the circuit.
U.S. Pat. No. 6,946,999 entitled TUNING TABS FOR A MICROSTRIP ANTENNA teaches a microstrip antenna tunable over a specified frequency range by the addition or removal of eight tuning tabs. While this means of tuning yields a relatively smooth control over the desired frequency range, two precisely aligned printed circuit boards are required to realize the device.
U.S. Pat. No. 6,759,917 entitled METHOD AND APPARATUS FOR ADJUSTING IMPEDANCE OF MATCHING CIRCUIT teaches a tuning method for a matching network, involving cutting and removing portions of stubs extending from signal traces. While this tuning method may permit extremely fine resolution, it directly affects the characteristics of the signal traces through the stubs and requires relatively complex equipment in order to make the necessary cuts in the stubs.
U.S. Pat. No. 6,005,519 entitled TUNABLE MICROSTRIP ANTENNA AND METHOD FOR TUNING THE SAME teaches a patch antenna with multiple smaller patches being either connected or disconnected from the main patch and from each other in order to control the effective length and width of the antenna. While this tuning method provides good control over bandwidth, resonant frequency and wave polarization, it does not permit contactless tuning of a microstrip with optimized dimensions.
U.S. Pat. No. 5,982,251 entitled TUNER FOR RADIO FREQUENCY TRANSMISSION LINES teaches a length of coaxial line with a single movable capacitive stub mounted alongside. The capacitive stub is micrometer-controlled for radial and axial position relative to the center conductor and interacts with the center conductor through a slit in the outer shield, thus providing tuning capability. This tuning method is not applicable to printed circuit boards and requires complex mechanical devices to be permanently incorporated into the tuned element.
So far, none of the existing T-junction designs intended for high power, low loss and highly coherent applications have been implemented in a microstrip technology with tuning capabilities for amplitude unbalance. Furthermore, none of the existing T-junction designs provide a tunable, low loss, high power handling and highly coherent T-junction (lossless power splitter/combiner) implemented in microstrip technology, where contactless tuning stubs are employed and where the tuning stub shape has been carefully engineered to maximize tuning capabilities while minimizing voltage breakdown phenomena.
A higher-performing tunable microstrip and T-junction design would rely on removable contactless tuning stubs wherein during tuning stub removal, the signal trace would not be touched and thus the carefully optimized dimensions of the signal trace would not be changed. To prevent contactless tuning stubs in a high voltage environment from experiencing voltage breakdown, an optimized shape would prevent voltage breakdown below 1.3 kV. While employing this optimized shape, the tuning stubs would provide a 0.4 dB amplitude unbalance tuning range to the T-junction circuit, and a tuning resolution of at least 0.02 dB. A higher-performing tunable microstrip and T-junction design would also make it feasible to produce large volumes of such devices both cost-effectively and with tightly controlled specs.